1. Field of the Invention
This invention relates to a temperature-compensated, solid-state voltage reference.
2. Description of the Related Art
Stable voltage references traditionally called "bandgap references" are commonly used in a wide variety of applications, including telecommunications. These references typically combine a small voltage, which is directly proportional to absolute temperature, with a larger voltage, which has a negative temperature coefficient. The two voltages are produced by two different diodes operating at current densities typically in the range of 10:1. The voltage difference between the two is then amplified by a DC amplifier with a gain that is established by the ratio of a polysilicon resistor divider. The goal of this voltage combination is to produce a substantially constant reference voltage over a wide temperature range.
There are, however, several sources of problems one encounters when trying to realize accurate, stable voltages using these conventional temperature-compensating voltage references. For example, the small difference between the voltages is directly proportional to absolute temperature. This voltage difference is typically only about 60 mV, but it varies approximately 0.2 mV per degree Celsius.
Because the voltage that is proportional to absolute temperature is small, the initial amplifier offset voltage can produce large changes in the reference voltage. For example, a 1.2 mV change in amplifier offset voltage can produce a 10 mV change in the initial reference voltage. In addition, small variations in the amplifier offset voltage or stress-induced changes in the polysilicon resistor divider ratio can produce large changes in the reference voltage.
Other sources of error include small variations in temperature between the diode elements themselves. For example, a temperature difference of less than 0.5.degree. C. between diode elements or amplifier input elements can produce a voltage difference of 1 mV at the reference output. Such errors can be significant, particularly during the time after switching from standby to full power in the chip that incorporates the reference.
A typical design goal for a voltage reference is the production of a device with better than one percent accuracy over a wide temperature range. Despite considerable research effort, only a few manufacturers of integrated circuits have been able to obtain such accuracy. Even where this accuracy has been achieved, however, it has typically been necessary to add complex circuits to the basic design discussed above. This, in turn, has required considerable chip area in order to compensate for several undesirable effects in the basic reference circuit. These circuits commonly include, for example, trimming circuits capable of adjusting the reference voltage after wafer probe and packaging.
In addition to increased chip area, these designs normally also require significant testing time to adjust the reference voltage. Unfortunately, reference voltage adjustments made at the wafer probe do not generally hold through the packaging process. Stress induced by the packaging process typically causes an accuracy in the voltage reference of better than one percent to become worse than one percent after packaging.
What is needed is therefore a voltage reference that provides the desired accuracy not only in theory but in practice, even after packaging, that does not require complex additional circuitry or long testing periods, and that can be implemented using easily calibrated and matched components.